Horneber has updated their website
and is now listing 408 CT (and 338 LM) brass. I contacted Huntingtons (their main US importer) and was advised, "We carry some Horneber cases, but not those calibers. We have to order a minimum of 1000 cases in each caliber."
So...if "you" (we) are willing to get something like a "group buy" going, Huntingtons can get the ball rolling. NOTE: Horneber is notorious for being VERY expensive and VERY slow in making their deliveries. That noted, listing those two cartridges certainly wasn't done for drill.